The Macaca irus (fascicularis) monkey has been established as a model for the study of CAHD, generalized atherosclerosis and peripheral vascular disease. On a cholesterol-corn oil diet this species predictably develops severe stenotic lesions of the proximal coronary and the ilio-femoral arteries. The development of the stenoses was documented in vivo by serial angiography with good correlation to post mortem morphological and biochemical findings. The severe stenoses of the coronary arteries were associated with ischemic EKG-changes and with subendocardial necroses and fibroses; those of the ilio-femoral arteries resulted in impaired blood flow to the limb. The arterial lesions consisted mainly of massively proliferated foam cells with some accumulation of collagen and calcium as well as alterations of the elastic tissue. By substituting butter for corn oil, fibrous plaques were induced instead of foam cell lesions. Currently, groups of monkeys on these diets have been placed on cholesterol- and fat-free diets to test to what extent the different types of lesions as well as their sequelae can regress. Other groups were continued on these diets to test what alterations of the lesions will thereby be induced and whether further luminal narrowing will lead to even more severe myocardial and peripheral disease.